The present invention relates to a control rod installed in a boiling water reactor (BWR), and particularly to a control rod using metal hafnium.
In a conventional boiling water reactor, boron carbide type control rods and hafnium type control rods have been used. With a boron carbide type control rod, a plurality of neutron absorber tubes filled with boron carbide powder are covered by U-shaped sheaths, and these are formed in a cruciform manner. Stainless steel (SUS) is used in the neutron absorber tubes and the U-shaped sheaths. In hafnium type control rods, metal hafnium (Hf) rods, plates or oval tubes are arranged in a cruciform manner. Boron carbide and Hf are used as neutron absorber material.
Hf does not have a large thermal neutron absorption cross section, but has a lot of peaks in the resonance energy region. This means that reactivity control ability can be maintained over a long period of time, so it is effective as a reactor control material. As conventional examples using hafnium, for example, a structure using hafnium in an upper portion and side portion of a BWR control rod is disclosed in Japanese patent publication No. Sho. 58-44237.
Hf has excellent corrosion resistance in high temperature water, which is why it is suitable as a material for reactors. However, since it is expected to maintain reactivity control ability for a long period of time, and also to be used for a long time, there here is a need to maintain high corrosion resistance over a long period of time in an in-pile oxidizing environment. Proposals relating to improving corrosion resistance of Hf are made with this type of situation in mind.
For example, Japanese patent laid-open publication No. Sho. 59-208044, discloses technology for improving nodular corrosion resistance by alloying niobium and zirconium (Zr) in specific proportions. Japanese patent laid-open publication No. Sho. 61-66188 discloses technology for prolonging the in-pile usage time by forming a diffuse cementation-coat of steel, chrome, nickel, niobium etc. on the outer surface of an Hf alloy including a small quantity of Zr. With these technologies of the related art, improvements in corrosion resistance can be expected, but they presume a large-scale manufacturing process or processing method.
On the other hand, as means for forming an oxidation cover layer on the surface of Hf, an autoclaving oxidation process has been considered. This method is generally used in zircalloy type fuel cladding pipes, and an oxidation cover layer is formed by processing in high temperature and high pressure vapor for a fixed time. However, with this method, processing facilities become large in size and the process time is long, which has a significant influence on the manufacturing process.
With the method disclosed in Japanese patent laid-open publication No. Sho. 61-66188 as well, if a permeation layer is formed by ion coating, there is need for an annealing process taking a number of hours after that. Accordingly, the influence in the manufacturing process can not be ignored in this case either.